..............................................................

Solar wind dynamic pressure andelectric field as the main factorscontrolling Saturn’s auroraeF. J. Crary1, J. T. Clarke2, M. K. Dougherty3, P. G. Hanlon3, K. C. Hansen4,J. T. Steinberg5, B. L. Barraclough5, A. J. Coates6, J.-C. Gerard7,D. Grodent7, W. S. Kurth8, D. G. Mitchell9, A. M. Rymer6 & D. T. Young1

1Southwest Research Institute, Culebra Road, San Antonio, Texas 78288, USA2Boston University, 725 Commonwealth Avenue, Boston, Massachusetts 02215,USA3Blackett Laboratory, Imperial College of Science and Technology, LondonSW7 2BZ, UK4The University of Michigan, Space Research Building, Ann Arbor, Michigan48109, USA5Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA6University College London, Mullard Space Science Laboratory, Surrey RH5 6NT,UK7Universite de Liege, 17 avenue du 6 aout, B4000 Liege, Belgium8Department of Physics and Astronomy, The University of Iowa, Iowa City,Iowa 52242, USA9Applied Physics Laboratory, Johns Hopkins University, 11100 Johns HopkinsRoad, Laurel, Maryland 20723, USA.............................................................................................................................................................................

The interaction of the solar wind with Earth’s magnetospheregives rise to the bright polar aurorae and to geomagnetic storms1,but the relation between the solar wind and the dynamics of theouter planets’ magnetospheres is poorly understood. Jupiter’smagnetospheric dynamics and aurorae are dominated by pro-cesses internal to the jovian system2, whereas Saturn’s magneto-sphere has generally been considered to have both internaland solar-wind-driven processes. This hypothesis, however, istentative because of limited simultaneous solar wind and mag-netospheric measurements. Here we report solar wind measure-ments, immediately upstream of Saturn, over a one-monthperiod. When combined with simultaneous ultraviolet imaging3

we find that, unlike Jupiter, Saturn’s aurorae respond strongly tosolar wind conditions. But in contrast to Earth, the main con-trolling factor appears to be solar wind dynamic pressure andelectric field, with the orientation of the interplanetary magneticfield playing a much more limited role. Saturn’s magnetosphereis, therefore, strongly driven by the solar wind, but the solar windconditions that drive it differ from those that drive the Earth’smagnetosphere.

In the case of Earth’s auroral activity and associated magneticsubstorms, the north–south component of the interplanetary mag-netic field is especially important. A southward interplanetarymagnetic field allows efficient dayside reconnection between theEarth’s and the interplanetary magnetic field, and is an importantdriver of substorms, whereas a northward field results in high-latitude reconnection and significantly weaker control of magneto-spheric dynamics by the solar wind1. Solar wind shocks driveauroral activity, regardless of the interplanetary magnetic fieldorientation, and the magnitude of this effect correlates well withthe solar wind speed and magnetic field strength. However, theinfluence of shocks is much more pronounced when the inter-planetary magnetic field is southward4.

In contrast, Jupiter’s rapid rotation and strong magnetic field,together with a large, internal plasma source from its satellite, Io,drive radial, outward transport of plasma and magnetosphericcurrents. These processes are the primary source of dynamics(ref. 2, and references therein) and the aurorae5. Despite this, thesolar wind does exert some influence on Jupiter’s magnetosphere6–9.On the basis of the size of the planet’s magnetosphere and theobserved ease with which its magnetopause is pushed inward and

outward by changing solar wind conditions, it has been suggestedthat dynamic pressure is the main solar wind driver, rather than theorientation of the interplanetary magnetic field10,11. Saturn, likeJupiter, is a rapidly rotating planet and has significant internalplasma sources. However, its magnetic field and internal plasmasources are weaker than Jupiter’s. Observations12 by the Voyagerspacecraft showed a significant degree of solar wind control,although the origin of these correlations was unclear. In the caseof the Voyager studies, kilometric radio emissions were used as aproxy for magnetospheric dynamics.

Between 10 and 30 January 2004, the Cassini spacecraft measuredthe solar wind upstream of Saturn’s magnetosphere while theHubble Space Telescope (HST) made observations of Saturn’sultraviolet aurorae roughly once every other day3. The Cassinidata discussed here come primarily from the CAPS instrument(Cassini Plasma Spectrometer Subsystem, providing ion and elec-tron data from 1 eV to 50 keV; ref. 13) and from the magnetometer.During this period, Cassini solar wind ion measurements werelargely continuous (81% duty cycle) and interrupted only byoccasional downlinks of data to Earth. Cassini also made measure-ments of energetic particle flux, local plasma waves and auroralradio emissions from Saturn14.

During this period, the Cassini spacecraft was at a range of(78 ^ 5) £ 106 km from Saturn, over the post-dawn side of theplanet, and (31 ^ 2) £ 106 km upstream. Although this distanceseems large for an upstream solar wind monitor, the Saturn–Sun–spacecraft angle was under 3.38. Studies of Cassini and Galileo data,obtained near Jupiter, indicate that a spacecraft may act as a viableupstream monitor when the planet–Sun–spacecraft angle is lessthan ,208 (ref. 15). Moving at nominally 500 km s21, it would takea spherically symmetric disturbance in the solar wind 17 h to travelfrom Cassini to Saturn.

Figure 1 shows the solar wind magnetic field, ion speed and

Figure 1 Solar wind conditions measured by Cassini. The top panel shows proton density,

the middle panel the flow speed, and the lower panel the magnetic field. The black line

shows the magnitude of the magnetic field. Bx is shown in blue, By in green and Bz in red.

The magnetic field is in planetary coordinates, a right-handed system with z parallel to the

axis of rotation (and magnetic moment), and the Sun in the þx/z half-plane.

letters to nature

NATURE | VOL 433 | 17 FEBRUARY 2005 | www.nature.com/nature720© 2005 Nature Publishing Group

proton density. The solar wind speed is characterized by periods ofgradual decrease followed by shocks on 15 and 25 January.Enhanced ion densities are associated with these shocks, as areenhancements in energetic particle flux and low frequency plasmawaves. Energetic particle, wave and magnetic field data indicateanother shock on 1 January, when the spacecraft orientation did notallow direct measurements of core solar wind protons. Followingthe 25 January shock, the plasma density remained high andvariable until the end of the month, the magnetic field strengthwas enhanced and several subsequent shocks were encountered.These solar wind disturbances are consistent with corotatinginteraction regions. A bimodal solar wind structure as observedhere is expected during the declining phase of the solar cyclemagnetic sector boundaries, with a reversal of the dominant B y

component that occurred on 17 and 26 January. (B y indicates the ycomponent of the magnetic field; see Fig. 1 legend for coordinatesystem.) During the month of observations, the north–southcomponent of the interplanetary magnetic field was weak, and thefield was primarily in the east–west direction.

We have used Cassini measurements to estimate conditions atSaturn with a one-dimensional, numerical magneto-hydrodynamicmodel16. This model assumes spherical symmetry and propagatesthe measurements outward. It accounts for the fact that shockspropagate faster than the ambient flow speed, as well as for anyevolution of the solar wind during the final 0.25 AU of its trip toSaturn. As the model assumes spherical symmetry, the three-dimensional structure of the solar wind may cause errors in themodelled propagation time. We expect that our estimated arrivaltimes may be systematically late by up to roughly 10 h, but cannotbe more exact owing to the unconstrained, three-dimensionalstructure of the solar wind.

According to this model, the shocks observed on 15 and 25January reached Saturn at 12:45 UTC, 16 January, and 08:00 UTC,

26 January. Figure 2 shows the propagated plasma speed and themagnetic field, along with four of the HST images. The first image,32 h before the arrival of the 25 January shock, is typical of otherimages taken during undisturbed solar wind conditions. The secondimage, 11 h after the shock arrival (þ11 h), shows a brightened andpartially filled auroral oval. In the third image, at þ42 h, the oval hasshifted to higher latitudes while remaining atypically bright. In thefinal image, at þ107 h, the aurora has returned to a more typicalstate. The response to the 15 January shock appears similar,although both the shock and the auroral response were weaker,and the first image obtained after the shock was at þ40 h (ref. 3).

Figure 3 shows a plot of propagated solar wind dynamic pressureagainst the input auroral power, estimated from the total ultravioletbrightness3. There is a very good correlation between auroral powerand both the solar wind dynamic pressure, ru2 (r ¼ 0.93), and theconvection electric field, u £ B (r ¼ 0.97). (Here r is the massdensity, u the flow velocity, r the linear correlation coefficient, u theflow velocity vector, and B the magnetic field vector.) These highcorrelation coefficients are, in part, a statistical artefact of the 25January event, which produced the two unusually high-powerpoints. However, this event is not the sole source of the correlations.Using only those observations with an auroral power below5 £ 1010 W (that is, using only the lower-power half of the measure-ments and thus excluding the unusual events), we obtain correlationcoefficients of 0.62 and 0.92, respectively.

Correlations with auroral power also show a lack of control by thedirection of the interplanetary magnetic field. In the case of Earth’smagnetosphere, the dynamics correlate well with a gated solar windelectric field, u £ B cos4 (v/2), where v is the projected anglebetween the interplanetary magnetic field and the planet’s magneticmoment17. The v term accounts for efficient, low-latitude reconnec-tion when the interplanetary magnetic field is parallel to the planet’smagnetic moment. For the January measurements, the correlationcoefficient with this parameter is 0.74. This is a significantly lowervalue than that obtained without the v term, suggesting that theorientation of the interplanetary magnetic field is not a controllingfactor.

At the same time, the interplanetary magnetic field duringJanuary was primarily in the y direction, while the north–southcomponent was weak and varying, with v ¼ 91 ^ 328. An alterna-tive explanation is that the cos4 gating function is not well-suited forthe interaction between the solar wind and Saturn’s magnetosphere.This relation was derived for the Earth and for a wide variety of solarwind magnetic field orientations, including the predominantlysouthward conditions that produce a strong magnetospheric

Figure 3 Correlation between HST-derived auroral input power and solar wind conditions.

Triangles show the solar wind (SW) dynamic pressure, ru 2; diamonds show the solar

wind convection electric field, ju £ Bj.

Figure 2 Comparison between HST images and solar wind conditions propagated to

Saturn for the period 25–30 January 2004. For details of the HST images, see ref. 3.

Thick blue vertical lines indicate the times of the four HST images shown above. The solar

wind conditions are presented in the same format as Fig. 1, but density and the Bxcomponent are omitted for clarity. In one-dimensional, time-dependent magneto-

hydrodynamics with a uniform upstream boundary condition, the x component cannot be

accurately modelled. The modelled propagation times may be systematically long by as

much as 10 h. Non-radial orientation of the solar wind structures, typical of corotating

interaction regions, could cause them to reach Saturn roughly five hours earlier than

spherically symmetric calculations would suggest. Solar rotation and the 3.38 difference

between the spacecraft’s and Saturn’s heliocentric longitude could also move arrival

times earlier by approximately 6 h. Overall, the estimated arrival times may be

systematically late by up to roughly 10 h.

letters to nature

NATURE | VOL 433 | 17 FEBRUARY 2005 | www.nature.com/nature 721© 2005 Nature Publishing Group

response. It is possible that the most appropriate gating function forvalues of v near 908 differs from the cos4 relation best suited for thefull range of v.

It is interesting to note that the January observations occurrednear southern summer, and that the average interplanetary mag-netic field is almost purely azimuthal at Saturn’s distance from theSun. As a result, the dominant component of the field wasperpendicular to Saturn’s magnetic moment. Solar wind controlof Saturn’s magnetosphere could be significantly different near theequinoxes, when Saturn’s axial tilt results in a 64.58 angle betweenthe dominant, azimuthal field component and Saturn’s magneticmoment. This would be a manifestation of a well-documentedphenomenon at Earth18, but made more extreme owing to thealignment between Saturn’s magnetic moment and spin axis.

Overall, the January Cassini and HSTmeasurements indicate thatSaturn’s magnetosphere, although similar in some respects to thoseof the Earth and Jupiter, is not simply an intermediate case. Itresembles Earth’s magnetosphere in that its auroral dynamics arestrongly driven by solar wind conditions. But it differs from Earth’sinteraction with the solar wind by virtue of an evidently weakinfluence of the direction of the interplanetary magnetic field. Thismay either be a characteristic of Saturn’s magnetosphere, or be aresult of differences between the solar wind at 10 AU and 1 AU

(specifically, the weaker north–south component of the solarwind magnetic field). The latter possibility suggests that Saturn’smagnetospheric response to the solar wind may be primarily drivenby solar wind shocks, a process that is observed at the Earth2 butgenerally overshadowed by the strong response of Earth’s magneto-sphere to the orientation of the interplanetary magnetic field. Thisapparent insensitivity to the magnetic field orientation and sensi-tivity to solar wind dynamic pressure and/or motional electric fieldmake Saturn’s magnetosphere resemble that of Jupiter more thanthat of Earth. In contrast to Jupiter, where the influence of the solarwind is weak and the aurorae are largely due to internal processes,the solar wind plays a controlling role in Saturn’s auroraldynamics. A

Received 9 September; accepted 27 December 2004; doi:10.1038/nature03333.

1. Dungey, J. W. Interplanetary magnetic field and the auroral zone. Phys. Rev. Lett. 6, 47–48 (1961).

2. Hill, T. W., Dessler, A. J. & Goertz, C. K. in Physics of the Jovian Magnetosphere (ed. Dessler, A. J.)

353–394 (Cambridge Univ. Press, Cambridge, 1983).

3. Clarke, J. T. et al. Morphological differences of Saturn’s ultraviolet aurorae and those of Earth and

Jupiter. Nature doi:10.1038/nature03331 (this issue).

4. Meurant, M., Gerard, J.-C., Blockx, C., Hubert, B. & Coumans, V. Propagation of electron and proton

shock-induced aurora and the role of the interplanetary magnetic field and solar wind. J. Geophys. Res.

109, A10210, doi:10.1029/2004JA010453 (2004).

5. Cowley, S. W. H. & Bunce, E. J. Origin of the main auroral oval in Jupiter’s coupled magnetosphere-

ionosphere system. Planet. Space Sci. 49, 1067–1088 (2001).

6. Zarka, P. & Genova, F. Low frequency jovian emissions and solar wind magnetic sector structure.

Nature 306, 767–768 (1983).

7. Desch, M. D. & Barrow, C. H. Direct evidence for solar wind control of Jupiter’s hectometric

wavelength radio emission. J. Geophys. Res. 89, 6819–6823 (1984).

8. Gurnett, D. A. et al. Control of Jupiter’s radio emissions and aurorae by the solar wind. Nature 415,

985–987 (2002).

9. Kivelson, M. G. & Southwood, D. J. First evidence of IMF control of Jovian magnetospheric boundary

locations: Cassini and Galileo magnetic field measurements compared. Planet. Space Sci. 51, 891–898

(2003).

10. Southwood, D. J. & Kivelson, M. G. A new perspective concerning the influence of the solar wind on

the Jovian magnetosphere. J. Geophys. Res. 106, 6123–6130 (2001).

11. Cowley, S. W. H. & Bunce, E. J. Modulation of Jupiter’s main auroral oval emissions by solar wind

induced expansions and compressions of the magnetosphere. Planet. Space Sci. 51, 57–79 (2003).

12. Desch, M. D. Evidence for solar wind control of Saturn radio emission. J. Geophys. Res. 87, 4549–4554

(1982).

13. Young, D. T. et al. Cassini plasma spectrometer investigation. Space Sci. Rev. (in the press).

14. Kurth, W. S. et al. An Earth-like correspondence between Saturn’s auroral features and radio emission.

Nature doi:10.1038/nature03334 (this issue).

15. Hanlon, P. G. et al. On the evolution of the solar wind between 1 and 5AU at the time of the Cassini-

Jupiter flyby: multi spacecraft observations of ICMEs including the formation of a merged interaction

region. J. Geophys. Res. 109, A09S03, doi:10.1029/2003JA010112 (2004).

16. Toth, G. General code for modeling MHD flows on parallel computers: Versatile advection code.

Astrophys. Lett. Commun. 34, 245–250 (1996).

17. Akasofu, S.-I. The solar wind-magnetosphere energy coupling and magnetospheric disturbances.

Planet. Space Sci. 28, 495–509 (1980).

18. Russell, C. T. & McPherron, R. L. Semiannual variation of geomagnetic activity. J. Geophys. Res. 78,

92–108 (1973).

Acknowledgements F.J.C., B.L.B., J.T.S. and D.T.Y. were supported by NASA through a Jet

Propulsion Laboratory contract with SWRI; D.G.M., K.C.H. and W.S.K. were supported through

other NASA/JPL contracts; P.G.H. was supported by a PPARC-UK quota studentship; and J.C.G.

and D.G. were supported by the Belgian Foundation for Scientific Research (FNRS) and the

PRODEX program of the ESA. This work is based on observations with the NASA/ESA Hubble

Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the

AURA, Inc., for NASA.

Competing interests statement The authors declare that they have no competing financial

interests.

Correspondence and requests for materials should be addressed to F.J.C. (fcrary@swri.edu).

..............................................................

An Earth-like correspondencebetween Saturn’s auroralfeatures and radio emissionW. S. Kurth1, D. A. Gurnett1, J. T. Clarke2, P. Zarka3, M. D. Desch4,M. L. Kaiser4, B. Cecconi1, A. Lecacheux3, W. M. Farrell4, P. Galopeau5,J.-C. Gerard6, D. Grodent6, R. Prange3, M. K. Dougherty7 & F. J. Crary8

1Department of Physics and Astronomy, The University of Iowa, Iowa City,Iowa 52242, USA2Boston University, 725 Commonwealth Avenue, Boston, Massachusetts 02215,USA3Space Research Department, Observatoire de Paris, 92195 Meudon, France4NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771, USA5CETP/UVSQ, 78140 Velizy, France6LPAP, Universite de Liege, allee du 6 aout, 17, B-4000 - Liege, Belgium7Blackett Laboratory, Imperial College of Science and Technology, LondonSW7 2BZ, UK8Southwest Research Institute, Culebra Road, San Antonio, Texas 78288, USA.............................................................................................................................................................................

Saturn is a source of intense kilometre-wavelength radio emis-sions that are believed to be associated with its polar aurorae1,2,and which provide an important remote diagnostic of its mag-netospheric activity. Previous observations implied that theradio emission originated in the polar regions, and indicated astrong correlation with solar wind dynamic pressure1,3–7. Theradio source also appeared to be fixed near local noon and at thelatitude of the ultraviolet aurora1,2. There have, however, been noobservations relating the radio emissions to detailed auroralstructures. Here we report measurements of the radio emissions,which, along with high-resolution images of Saturn’s ultravioletauroral emissions8, suggest that although there are differences inthe global morphology of the aurorae, Saturn’s radio emissionsexhibit an Earth-like correspondence between bright auroralfeatures and the radio emissions. This demonstrates the univer-sality of the mechanism that results in emissions near theelectron cyclotron frequency narrowly beamed at large anglesto the magnetic field9,10.

The first studies of Saturn’s primary radio emission (Saturnkilometric radiation, SKR) were by the Voyager spacecraft in theearly 1980s1,3. This emission had some features that were more likethose of Earth than of Jupiter: the SKR source appears to be fixed inlocal time, favouring a region near local late morning or noon, butvarying with source latitude2. It is also strongly correlated with solarwind parameters, such as the dynamic pressure7. However, the SKRalso has some features that are more like those of Jupiter than ofEarth: it was found to be strongly modulated by planetary rotation,despite the fact that models of Saturn’s magnetic field are axisym-metric11. This has suggested an as-yet-unobserved ‘active sector’ orpossible ‘magnetic anomaly’ rotating with the planet12,13.

The measurements shown here are primarily from the radio

letters to nature

NATURE | VOL 433 | 17 FEBRUARY 2005 | www.nature.com/nature722© 2005 Nature Publishing Group